In the drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The rapid advance toward finer pattern rules is grounded on the development of a projection lens with an increased NA, a resist material with improved performance, and exposure light of a shorter wavelength. In particular, the change-over from i-line (365 nm) to shorter wavelength KrF laser (248 nm) brought about a significant innovation, enabling mass-scale production of 0.18 micron rule devices. To the demand for a resist material with a higher resolution and sensitivity, acid-catalyzed chemical amplification positive working resist materials are effective as disclosed in U.S. Pat. No. 4,491,628 and U.S. Pat. No. 5,310,619 (JP-B 2-27660 and JP-A 63-27829). They now become predominant resist materials especially adapted for deep UV lithography.
Resist materials adapted for KrF excimer lasers enjoyed early use on the 0.3 micron process, passed through the 0.25 micron rule, and currently entered the mass production phase on the 0.18 micron rule. Engineers have started investigation on the 0.15 micron rule, with the trend toward a finer pattern rule being accelerated. A wavelength change-over from KrF to shorter wavelength ArF laser (193 nm) is expected to enable miniaturization of the design rule to 0.13 μm or less. Since conventionally used novolac resins and polyvinylphenol resins have very strong absorption in proximity to 193 nm, they are difficult to use as the base resin for resists. To ensure transparency and dry etching resistance, some engineers investigated acrylic and alicyclic (typically cycloolefin) resins as disclosed in JP-A 9-73173, JP-A 10-10739, JP-A 9-230595 and WO 97/33198.
Among others, a focus is drawn on (meth)acrylic resin base resists featuring a high resolution. One of the (meth)acrylic resins proposed thus far is a combination of (meth)acrylic units having methyladamantane ester as acid labile group units with (meth)acrylic units having lactone ring ester as adhesive group units as disclosed in JP-A 9-90637. Acid labile groups of exo form are described in U.S. Pat. No. 6,448,420 (JP-A 2000-327633). These groups have so high an acid lability and require a low level of activation energy for acid elimination, affording a high resolution and low dependence on post-exposure bake (PEB). Norbornane lactone is also proposed as an adhesive group having enhanced etching resistance as disclosed in JP-A 2000-26446 and JP-A 2000-159758.
Of the outstanding tasks associated with the ArF lithography, it is desired to minimize the mask error factor (MEF) and to prevent pattern collapse during development. When the line width of the mask is changed 1 nm, the MEF is an index of a change (in nm) in the line width of the corresponding resist. The MEF requirement becomes stricter as the pattern rule becomes finer. On the other hand, the pattern collapse is a phenomenon that the resist pattern collapses by a capillary force during the spin development step. One factor causing the pattern collapse is swelling during development. While the polyhydroxystyrene used as the resist for the KrF lithography, in which the phenol moiety is a weak acidic group and has an appropriate alkali solubility, is resistant to swelling, the ArF lithography uses polymers containing hydrophobic cycloaliphatic groups, which must be dissolved using carboxylic acids having a high acidity, leading to a likelihood for the polymers to swell during development.
The development performance of resists can be quantified by the quartz crystal microbalance (QCM) technique. The quantity of swell during development is reported in Proc. SPIE Vol. 3999, p 2 (2000). Although the swelling of a film being developed could not be observed by the prior art film thickness measurement relying on optical interference, the QCM technique designed to electrically measure any change of film weight enables to observe any weight increase of swollen film. The cited reference discusses the swelling of ArF resists based on cycloolefin polymers. Substantial swells are observed when carboxylic acid is used as the adhesive group.
For the F2 lithography, resists using hexafluoroalcohol have been studied. As used herein, the term “hexafluoroalcohol” refers to an alcohol having total six fluorine atoms on the carbon atoms located adjacent the alcoholic carbon. It is reported in J. Photopolym. Sci. Technol., Vol. 16, No. 4, p 523 (2003) that hexafluoroalcohol has an acidity approximate to that of phenol and is least swollen in a developer liquid. Also known are polynorbornene having hexafluoroalcohol and α-trifluoromethyl acrylate having hexafluoroalcohol pendants. It was reported how these polymers perform when exposed to ArF excimer laser light.